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- Aerosole (10)
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Institute
BTU
Aerosolverdünnung
(2001)
Einfluss der Oberflächenbeschaffenheit des Filtermediums auf den spe-zi-fischen Kuchenwiderstand
(2001)
Einfluss der Oberflächenbeschaffenheit des Filtermediums auf den spezifischen Staubkuchenwiderstand
(2001)
Neural Network Recongition of Particle Size Distributions from Ultrasonic Extinction Measurements
(1995)
Probleme mit dem Filter
(2003)
Characteristics of a new aerosol dilution system for volumetric flow rates between 0,2 and 1 l/min
(2004)
In order to help answering the question “Which electrode design is the best?”, the precipitation performance of a tubular ESP was determined for different electrode designs with special attention to specific energy consumption. Experiments were conducted in a wire-tube arrangement with a high loading of liquid submicron particles causing strong space charge effects, also known as corona quenching. The experiments are supplemented by numerical simulation results showing the effect of residence time and power consumption on separation efficiency.
Charge transfer between single highly resistive particles: Experiments and Numerical Simulation
(2019)
Extension of the One-Dimensional Turbulence model towards electrohydrodynamic variable density flows
(2019)
The removal of high-resistivity dusts from gas flows using electrostatic precipitators (ESPs) is challenging and requires counter-measures to suppress back-corona and subsequent particle re-entrainment. The critical dust layer thickness for the occurrence of back-corona may vary by several orders of magnitude depending on the specific dust resistivity, dust layer porosity, particle size, humidity and operating temperature. Despite the difficulties at elevated operating temperatures, processes involving highly resistive dusts (e.g. oxidic materials) may benefit significantly from utilizing high temperature ESPs (HT-ESPs). This is because the specific resistivity of materials like SiO2, Al2O3 and other oxides decreases below the critical value of 10^11 Ω*cm at higher temperatures. Currently, the separation of oxidic nanoparticle products relies on cloth filters which require exhaust gas quenching prior to dust removal. Using heat exchangers instead to recover excess heat is not feasible due to severe scale formation. Therefore, the development of a reliable HT-ESP would allow heat exchange at clean gas conditions reducing operating costs of the overall process.
During preliminary experiments for our ongoing investigation of HT-ESPs, the behaviour of a flame-generated SiO2 aerosol was studied at 200°C in a wire-tube configuration. In order to reach a better understanding of the back-corona, measurements of the particle size distribution, particle charge distribution and electric current were conducted for both polarities.
Strong effects are found even though the dust layer thickness is a few μm only. While the separation efficiency decreases rapidly after a critical dust layer thickness is reached, the current uptake increases significantly (up to a factor of 20), and a large number of inversely charged particles is found in the clean gas. These effects may indicate a large-scale back-corona. A detailed discussion will be given on the basis of aerosol size and mobility distributions, SEM images and resistivity measurements of the dust layer. We gratefully acknowledge the financial support for this project (EFRE-StaF 23035000) by the European Fund for Regional Development.
Electrostatic charging and deposition of a liquid nano-aerosol was studied in dry air and in technical (3.6 % O2) nitrogen. The experiments have shown that electronic charging of aerosols can be important in technical scale electrostatic precipitators (ESPs). Already at operation voltages just slightly above the corona onset voltage, the contribution of the free electrons to the overall current is estimated to be around 50 %. Due to the high temperature of free electrons, diffusion charging by free electrons allows to reach exceptionally high particle charge and extremely high precipitation efficiency.
A strongly simplified theoretical model was developed, which gives a good prediction of particle charge based on averaged values of particle diameter, current density, electric field strength, electron temperature and residence time. The ion mobilities were determined by fitting the current-voltage characteristics with a modified Townsend (Monrolin et al. 2018) equation and are significantly higher than the values typically used to describe diffusion charging in air. This may be ascribed to the very dry gas phase and the short average lifespan of the ions under ESP conditions, which is in the order of 1 ms.
From the practical point of view, electronic charging might be relevant in a number of technical applications, including high temperature ESPs, ESP applications in dry and oxygen-free gases and pulsed corona systems.
We gratefully acknowledge the financial support for this project (EFRE-StaF 23035000) by the European Fund for Regional Development.
Monrolin, N., Praud, O., Plouraboué, F. 2018. Revisiting the positive DC corona discharge theory: Beyond Peek's and Townsend's law. Phys. Plasmas 25, 063503; doi: 10.1063/1.5031780.
Enhanced heat and mass transfer due to electrohydrodynamically induced flow in electrostatic precipitators provides a potential opportunity for versatile new applications in the process industry. After a short review of the physical fundamentals it is explained why measurements of the pressure gradient play an important role in the design principles for this technology. Its external influence by operating parameters like voltage polarity and aerosol conditions will also be shown. In the second part, the feasibility of enhanced mass and heat transfer is demonstrated based on laboratory measurements. In combination with the previously discussed pressure gradient measurements, methods for simplified engineering concepts like equivalent mean flow velocities or turbulent diffusion coefficients are shown. The topic is rounded off by outlining a potential use in decentralized power generation.
Zur Förderung einer nachhaltigeren Wertschöpfungskette werden im Rahmen der Energiewende ständig neue Verfahren und Konzepte zu deren Umsetzung entwickelt. Eines dieser Konzepte befasst sich mit der Zusammenführung bislang entkoppelter Prozessschritte in Gasreinigungssystemen aus der Energie- und Anlagentechnik. Aus den daraus resultierenden Einsparungen bei den Investitionskosten profitieren nicht nur die klassischen Energieerzeugeuer und Produktionsanlagen in der chemischen Industrie, sondern es werden auch neue Anreize zur dezentralen Energieeinspeisung von kleineren und mittelständischen Unternehmen gesetzt.
Elementarer Bestandteil dieser Gasreinigungssysteme sind häufig Elektroabscheider, welche weit verbreitet im Einsatz sind um Abgase von Stäuben bzw. Aerosolen zu befreien, oder um staubförmige Wertprodukte aus Gasströmungen abzuscheiden. Die in Elektroabscheidern prinzipbedingt vorhandenen elektro¬¬hydro¬dyna¬mischen Strömungen, auch als elektrischer Wind bezeichnet, sollen nun systematisch genutzt werden, um neben der Aerosolabscheidung gleichzeitig Aufgaben des Wärme- und Stoffaustauschs zu lösen.
Inhalte der Untersuchungen
Zur Validierung der Machbarkeit dieser neuen Techniken wurde der Einfluss des elektrischen Windes auf die turbulente Quervermischung, anhand des verbesserten Wärme- und Stofftransports, sowie des Druckverlusts im Rohrelektroabscheider getestet. Bei den Untersuchungen wurden unterschiedliche Strömungsgeschwindigkeiten, Elektrodengeometrien und Betriebsspannungen, sowie die Anwesenheit von Aerosol berücksichtigt. Ein Teil der Ergebnisse diente bereits als Validierungsgrundlage für numerische Simulationen mit einem neuartigen Turbulenzmodell [1].
[1] Medina, J., Schmidt, H., Riebel, U., Application of the one-dimensional turbulence model for electrohydrodynamic variable density flows, 17th European Turbulence Conference, 3-6 September 2019, Torino, Italy
Gas-phase particle separation at significantly elevated temperatures is a scope since heat recovery units might be operated with higher efficiency and lower maintenance costs when fouling by particulate deposits could be reduced. However, this proves difficult for most separation techniques for numerous reasons (particle size limitations, material limitations, pressure difference limitations). While the upper temperature limit for standard electrostatic precipitators (ESP) operated at ambient pressure and with negative polarity is around 400°C, positive polarity theoretically should allow an operation with temperatures well above 600°C. However, there seems to be very little experience with high temperature ESPs even on the lab scale or small pilot scale.
To close this apparent gap in electrostatic precipitation research our current project aims to investigate the characteristics of a SiO2 aerosol and the capabilities of an ESP dealing with said SiO2 aerosol at 600°C and above. The aerosol is produced by combustion of silicone oil (HMDSO) in a propane flame which leads to particle size distributions ranging from below 10 nm up to 400 nm. The median value of the distribution and the total number concentration is a function of the gaseous HMDSO amount introduced into the flame. Increasing the HMDSO supply shifts the median to larger particle diameters but also reduces the total number concentration, which is an interesting phenomenon to be looked into in more detail during the project since coagulation cannot explain this on its own. The observed high number concentrations at low HMDSO feed rates may involve thermal rebound which needs further investigation. Additionally, the bipolar charge distribution typical for flame-generated aerosols has to be considered when deriving an explanation for the observations.
The next steps will be a more detailed characterization of the produced SiO2 aerosol and the operation of a high temperature ESP in wire-tube geometry.
Positive and negative corona discharges were studied at atmospheric pressure in a tube-wire geometry with a tube diameter of 150 mm. Current-voltage characteristics (CVCs) were measured both in air and in flue gas produced with a liquefied petroleum gas (LPG) burner, covering temperatures from ambient up to 1073 K. Corona operation was stable over the whole temperature range with positive polarity and up to 973 K with negative polarity. Based on a detailed analysis of the original CVC measurements, new insights on the temperature-dependent mobility of gas ions, on the occurrence of free electrons and on the electronic current contribution are presented. Deviations from the Townsend theory of CVCs can be ascribed to the lifetime-dependency of ion mobility caused by the formation of cluster ions. The contribution from free electrons is found to depend on temperature and tube radius.